1. Technical Field of the Invention
The present invention relates to multi-gap electric rotating machines which are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of the Related Art
There are electric rotating machines which are used in a narrow space and thus required to be flat in shape, such as an engine direct-coupled motor that is arranged between an engine and a transmission in a hybrid vehicle and a motor for a household appliance (e.g., a washing machine). Further, as a method of increasing the output torque of those electric rotating machines, there is known a double-gap structure which includes a radially inner magnetic gap and a radially outer magnetic gap.
Moreover, as double-gap electric rotating machines, there are known 1 stator-2 rotor electric rotating machines and 1 rotor-2 stator electric rotating machines.
The 1 stator-2 rotor electric rotating machines include a stator and inner and outer rotors. The stator includes a stator core and a stator coil mounted on the stator core. The inner rotor is disposed radially inside of the stator so as to be surrounded by the stator with a radially inner magnetic gap formed therebetween. The outer rotor is disposed radially outside of the stator so as to surround the stator with a radially outer magnetic gap formed therebetween.
However, the 1 stator-2 rotor electric rotating machines involve the following problems. First, though there is provided only single stator, the number of turns of the stator coil is large. Secondly, both the inner and outer rotors are thin and wide in shape. Moreover, the inner rotor is exposed on the radially inner side while the outer rotor is exposed on the radially outer side. Consequently, both the inner and outer rotors may be easily deformed, thereby generating loud noise during operation. Thirdly, since the stator is radially interposed between two rotating objects, i.e., the inner and outer rotors, it is difficult to securely fix the stator in the electric rotating machine.
On the other hand, the 1 rotor-2 stator electric rotating machines include a rotor and inner and outer stators. The inner stator is disposed radially inside of the rotor while the outer stator is disposed radially outside of the rotor.
For example, FIG. 13 shows a 1 rotor-2 stator in-wheel motor for a vehicle, which is originally disclosed in Japanese Patent Application Publication No. 2007-282331. As shown in the figure, this motor includes a rotor 20A, an inner stator 31A disposed radially inside of the rotor 20A, and an outer stator 33A disposed radially outside of the rotor 20A. Consequently, the rotor 20A is radially interposed between the inner and outer rotors 31A and 33A, and thus exposed neither on the radially inner side nor on the radially outer side. Moreover, both the inner and outer stators 31A and 33A can be securely fixed in the motor.
With the above configuration, however, it is necessary to wind two stator coils respectively for the inner and outer stators 31A and 33A and separately fix the inner and outer stators 31A and 33A to the wheel frame. Consequently, the assembly process of the motor is complicated, and it is difficult to secure high concentricity of the inner and outer stators 31A and 33A.
To solve the above problem, one may consider employing a fixing member to connect and thereby fix the inner and outer stators 31A and 33A together. However, in this case, the fixing member will be located axially outside of coil ends of the stator coils of the inner and outer stators 31A and 33A, thus increasing the overall axial length of the motor. Consequently, the motor may become no longer flat in shape, and thus it may become difficult to mount the motor in the wheel.